1. Field of the Invention
The present invention relates to an apparatus for preparing crystals of a rare earth element-activated, alkaline earth metal fluorohalide based stimulable phosphor precursor.
2. Description of the Related Art
A radiation image recording and reproducing method using a stimulable phosphor, which can replace conventional radiography, is known and described, for example, in Japanese Patent Application Laid-Open (JP-A) No. 55-12145. This method utilizes a radiation image conversion panel containing a stimulable phosphor (that is, an accumulative phosphor sheet). In this method, radiation transmitted through a subject or emitted from an object to be examined is absorbed into the stimulable phosphor of the panel, and the stimulable phosphor is excited by an electromagnetic wave (excitation light) such as visible light or infrared light in a time sequential manner so as to allow radiation energy accumulated in the stimulable phosphor to emit as fluorescent emission (stimulable emission). Subsequently, an electric signal is obtained by photoelectrically reading the fluorescence, and based on the obtained electric signal, a radiation image of the subject or object to be examined is reproduced as a visible image. The panel for which the reading has been completed is made ready for subsequent photographing after remaining images have been deleted therefrom. That is, the radiation image conversion panel can be used repeatedly.
The above-described radiation image recording and reproducing method has an advantage in that a radiation image having much information can be obtained using a radiation dosage much smaller than in conventional radiography using a combination of a radiographic film and an intensifying screen. Further, a radiographic film is consumed each time an image is produced in conventional radiography. In contrast, a radiation image conversion panel in the radiation image recording and reproducing method is advantageous in terms of the aspects of resource protection and economic efficiency due to reusability.
The stimulable phosphor is a phosphor which exhibits stimulable emission when excitation light is irradiated thereon after irradiation with radiation. For practical use, a phosphor which emits stimulable emission in a wavelength range from 300 to 500 nm by excitation light in a wavelength range from 400 to 900 nm is generally employed. An example of the stimulable phosphor conventionally used in radiation image conversion panels is a rare earth element-activated, alkaline earth metal fluorohalide based phosphor. The radiation image conversion panel used in the radiation image recording and reproducing method has a basic structure comprised of a support and a stimulable phosphor layer provided on the support surface. However, a support is not necessarily needed if the stimulable phosphor layer is self-supporting. The stimulable phosphor layer is usually comprised of a stimulable phosphor and a binder which contains and supports the phosphor in a state of dispersion. Further, there has been known stimulable phosphor layers which do not contain a binder and are comprised solely of a stimulable phosphor in a state of aggregates formed by vapor deposition or sintering. Furthermore, there has also been known radiation image conversion panels each having a stimulable phosphor layer in which a high polymer material is impregnated into gaps in the aggregates of stimulable phosphors. Any of the above-described stimulable phosphor layers exhibits the property of stimulable emission when irradiated with excitation light after having absorbed radiation such as X-rays. Accordingly, the energy in an amount proportionate to the amount of the radiation transmitted through a subject or emitted from an object to be examined is absorbed in the stimulable phosphor layer of the radiation image conversion panel, and a radiation image of the subject or the object to be examined is formed as an image of accumulated radiation energy in the radiation image conversion panel. The accumulation image can be released as stimulable emission by irradiation with excitation light, and the photoelectric reading of the stimulable emission and the conversion thereof into an electric signal make it possible to make the radiation energy accumulation image into a visible image.
The surface of the stimulable phosphor layer (which surface does not face the support) has generally a protective film comprised of a polymer film or a vapor-deposited film of an inorganic substance to thereby protect the stimulable phosphor layer from chemical deterioration or physical impact.
The above-described rare earth element-activated, alkaline earth metal fluorohalide based stimulable phosphor can be a stimulable phosphor excellent in practical terms for the reason that it has an excellent sensitivity and provides a radiation reproduction image having a high level of sharpness when it is used for a radiation image conversion panel. However, with the advance of practicability of a radiation image recording and reproducing method, there has increased a demand for further enhancement of the stimulable phosphor. Here, as a result of examination of the grain shape of rare earth element-activated, alkaline earth metal fluorohalide based stimulable phohsphors having been utilized, the present inventors have found that these phosphors are tabular. A conventionally known process for preparing a rare earth element-activated, alkaline earth metal fluorohalide based stimulable phosphor comprises the steps of dry-blending or blending by suspending in a water based medium, material compounds, which are an alkaline earth metal fluoride, an alkaline earth metal halide other than fluoride, a halide of a rare earth element, ammonium fluoride, and the like, firing these compounds after the addition of a sintering preventing agent if necessary, and pulverizing the fired product. Accordingly, in this conventional process, the pulverizing step after the firing step is substantially essential, and most of grains of the rare earth element-activated, alkaline earth metal fluorohalide based stimulable phosphor thus produced were tabular (hereinafter referred to merely as “tabular phosphor” on occasion).
However, in a stimulable phosphor layer obtained by applying a mixture of the above-described tabular-grained phosphor and a binder resin solution on a support and drying the coating, the tabular-grained phosphors tend to be arranged in such a manner that surfaces thereof are made parallel to the surface of the support, as illustrated in FIG. 3. When a radiation image is stored in a radiation image conversion panel having a stimulable phosphor layer in which the tabular-grained phosphors are arranged in the above-described manner and the panel is irradiated with excitation light, the excitation light and stimulable emission generated are apt to extend in a transverse direction (that is, a direction parallel to the surface of the support as shown in the horizontal arrow shown in FIG. 3). As a result, there arises a problem that the sharpness of a radiation reproduction image to be obtained is apt to deteriorate.
In order to prevent deterioration in the sharpness of a radiation reproduction image in the radiation image recording and reproducing method as described above, there is proposed a process in which a stimulable phosphor comprised of approximately cubic grains is used, as disclosed in JP-A No. 62-86086. However, the process for preparing a stimulable phosphor comprised of approximately cubic grains disclosed above cannot provide sufficient reproducibility of grain shape, grain size, and grain size distribution for industrial utilization.
Further, JP-A No. 7-233369 discloses a process for preparing a rare earth element-activated, alkaline earth metal fluorohalide based stimulable phosphor (hereinafter referred to simply as “phosphor” on occasion) having a tetradecahedral grain structure in which grain shape and grain aspect ratio are controlled. In a radiation image conversion panel having a stimulable phosphor layer in which a rare earth element-activated, alkaline earth metal fluorohalide based stimulable phosphor having a tetradecahedral grain structure (hereinafter referred to simply as “tetradecahedron-structured phosphor” on occasion) is provided, as illustrated in FIG. 4, the directionality of the tetradecahedron-structured phosphor is reduced. Accordingly, the undesirable transverse extension of the excitation light and the stimulable emission is lessened and the sharpness of a resultant radiographic reproduction image improves.
In the process for preparing the phosphor disclosed in JP-A No. 7-233369, ammonium halide is used as reactant mother liquor and an aqueous solution of barium halide and an aqueous solution of an inorganic fluoride salt are simultaneously added to the mother liquor so as to allow the reaction to proceed, thereby synthesizing a stimulable phosphor. However, the grains thus produced tend to have a high grain aspect ratio. On the other hand, although the aspect ratio can be made closer to 1 to a certain extent by adding barium halide to the reactant mother liquor in advance, this process is not satisfactory from the standpoint of controllability of grain shape, grain size, and grain size distribution.